1. Field of the Invention
The present invention relates to a process for producing a branched aldehyde represented by the following formula (1); ##STR3## wherein Y represents an acyl group of two or more carbon atoms; and X represents an acyloxymethyl group represented by --CH.sub.2 OY' (wherein Y' represents an acyl group of two or more carbon atoms), cyano group or an alkoxycarbonyl group. The branched aldehyde produced by the process of the present invention is useful as an intermediate for phamaceuticals and agricultural chemicals. For example, the branched aldehyde produced by the process of the present invention can be converted into an .alpha., .beta.-unsaturated aldehyde represented by the formula ##STR4## wherein X is the same as defined above, which unsaturated aldehyde is useful as an intermediate for vitamin As see Pure & Appl. Chem., 63, 45(1991); British Patent No. 1168639; Japanese Patent Application Publication No. Sho 60-9493, etc.! and zeatin, a plant hormone see U.S. Pat. No. 4,361,702!.
2. Related Art of the Invention
Processes for producing 1,2-diacetoxy-3-formylbutane, one of the branched aldehyde represented by the above formula (1), have been known, which comprises hydroformylation of 3,4-diacetoxy-1-butene in the presence of rhodium compounds (see U.S. Pat. No. 3,732,287 and German Patent Application Laid-open No. 2039078).
The U.S. Pat. No. 3,732,287 discloses that 1,2-diacetoxy-3-formylbutane can be produced in good yield through hydroformylation at an elevated temperature and an elevated pressure. The patent also discloses that the reaction temperature is preferably 60 to 120.degree. C., more preferably 80 to 105.degree. C. The patent further describes that the reaction pressure is generally 300 to 1200 atm., preferably 500 to 700 atm.
The German Patent Application Laid-open No. 2039078 discloses, in the Example 1, that 3,4-diacetoxy-1-butene was converted, by the hydroformylation at 600 atm. and 100.degree. C. using a rhodium catalyst, to a mixture of 2000 g of 2-methyl-3,4-diacetoxybutanal (identical with 1,2-diacetoxy-3-formylbutane) and 1700 g of 4,5-diacetoxypentanal.
As described in the German Patent Application Laid-open No. 2039078, 3,4-diacetoxy-1-butene is a compound with an olefinic carbon-carbon double bond at a terminal of the molecule, so the hydroformylation of the compound generally gives a mixture of 4,5-diacetoxypentanal, a linear aldehyde, and 1,2-diacetoxy-3-formylbutane, a branched aldehyde.
The U.S. Pat. No. 3,732,287 and the German Patent Application Laid-open No. 2039078 both require to carry out the hydroformylation at least at a pressure as high as 300 atm in order to produce 1,2-diacetory-3-formylbutane in good yield. Therefore, the methods disclosed in these documents require high cost for equipment durable at such high pressure as described above in order to carry out the method in an industrial scale, and consequently, the production cost of 1,2-diacetoxy-3-formylbutane is disadvantageously high.
The present inventors have made attempts to reduce the pressure for the hydroformylation of 1,3-diacetoxy-1-butene for industrial advantages, and found that the selectivity to 1,2-diacetoxy-3-formylbutane was lowered. For example, the ratio of 1,2-diacetoxy-3-formylbutane and 4,5-diacetoxypentanal was 40/60 (former/latter) in the resulting product when the hydroformylation was carried out at 100 atm. and 80.degree. C. using a rhodium carbonyl complex as a catalyst.